Entire solid lithium secondary battery

ABSTRACT

The present invention provides an entire solid lithium secondary battery comprising: a cathode; an anode; and a solid electrolyte layer disposed between the cathode and the anode. The solid electrolyte layer is formed of a Li (1-x) TaO 3  crystal (where 0.12≦x≦0.46) having a trigonal ilmenite crystal structure. This entire solid lithium secondary battery has a good charge-discharge property.

BACKGROUND

1. Technical Field

The present invention relates to an entire solid lithium secondarybattery.

2. Description of the Related Art

The entire solid lithium secondary battery is a lithium secondarybattery including an electrolyte layer formed of a solid electrolyte.The entire solid lithium secondary battery has higher safety and higherenergy capacity than a lithium secondary battery including a liquidelectrolyte containing a flammable solvent. The entire solid lithiumsecondary battery comprises a cathode, an anode, and a solid electrolytelayer. The solid electrolyte layer is disposed between the cathode andthe anode. A cathode active material layer included in the cathode is incontact with the solid electrolyte layer. The cathode active materiallayer contains a cathode active material capable of storing andreleasing lithium ions. Similarly, an anode active material layerincluded in the anode is in contact with the solid electrolyte layer.The anode active material layer also contains an anode active materialcapable of storing and releasing lithium ions. The lithium ions travelthrough the solid electrolyte layer. In other words, in the entire solidlithium secondary battery, the lithium ions migrate between the cathodeactive material layer and the anode active material layer through thesolid electrolyte layer in association with the redox reaction on thecathode and the anode. Due to this migration, the entire solid lithiumsecondary battery is charged and discharged.

Even when the solid electrolyte formed of an oxide is exposed to theair, the high stability and the high safety thereof are maintained. A.M. Glass et al., “Ionic conductivity of quenched alkali niobate andtantalite glasses”, Journal of Applied Physics, 49(9), 1978, pp.4808-4811 discloses that amorphous LiNbO₃ and amorphous LiTaO₃ have asubstantially equal lithium ion conductivity. Japanese PatentApplication laid-open Publication No. 2010-251257A discloses acrystalline solid electrolyte material formed of a complex containingLiNbO₃ and LiNb₃O₈ at a specific mixture ratio. Japanese PatentApplication laid-open Publication No. 2010-251257A further disclosesthat amorphous LiNbO₃ has a higher lithium ion conductivity thancrystalline LiNbO₃.

SUMMARY

The present invention provides an entire solid lithium secondary batterycomprising:

a cathode:

an anode; and

a solid electrolyte layer disposed between the cathode and the anode;

wherein

the solid electrolyte layer is formed of a Li_((1-x))TaO₃ crystal (where0.12≦x≦0.46) having a trigonal ilmenite crystal structure.

The present invention provides an entire solid lithium secondary batterycomprising a solid electrolyte layer having a higher lithium ionconductivity than a conventional LiTaO₃ solid electrolyte. This entiresolid lithium secondary battery has a good charge-discharge property.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of an entire solid lithium secondarybattery according to an embodiment.

FIG. 2A shows a cross-sectional view of one step included in a methodfor fabricating the entire solid lithium secondary battery according tothe embodiment.

FIG. 2B shows a cross-sectional view of one step, subsequent to the stepshown in FIG. 2A, included in the method for fabricating the entiresolid lithium secondary battery according to the embodiment.

FIG. 2C shows a cross-sectional view of one step, subsequent to the stepshown in FIG. 2B, included in the method for fabricating the entiresolid lithium secondary battery according to the embodiment.

FIG. 2D shows a cross-sectional view of one step, subsequent to the stepshown in FIG. 2C, included in the method for fabricating the entiresolid lithium secondary battery according to the embodiment.

FIG. 3 shows a wide-angle X-ray diffraction profile of the solidelectrolyte layer according to the inventive example 1.

FIG. 4 shows a wide-angle X-ray diffraction profile of the solidelectrolyte layer according to the comparative example 2.

DETAILED DESCRIPTION OF THE EMBODIMENT

Hereinafter, the present invention will be described with reference tothe drawings.

FIG. 1 shows a cross-sectional view of an entire solid lithium secondarybattery 1 according to an embodiment. The entire solid lithium secondarybattery 1 shown in FIG. 1 comprises a cathode 2, an anode 3, and a solidelectrolyte layer 4. The solid electrolyte layer 4 is interposed betweenthe cathode 2 and the anode 3. The cathode 2 comprises a cathodecollecting electrode 21 and a cathode active material layer 22 formedthereon. The cathode active material layer 22 is electrically connectedto the cathode collecting electrode 21 in series. In FIG. 1, the cathodeactive material layer 22 is in contact with the cathode collectingelectrode 21. The anode 3 comprises an anode collecting electrode 31 andan anode active material layer 32 formed thereon. The anode activematerial layer 32 is electrically connected to the anode collectingelectrode 31 in series. In FIG. 1, the anode active material layer 32 isin contact with the anode collecting electrode 31. The solid electrolytelayer 4 is disposed so that lithium ions can travel between the cathodeactive material layer 22 and the anode active material layer 32. In FIG.1, the front surface of the solid electrolyte layer 4 is in contact withthe cathode active material layer 22. The back surface of the solidelectrolyte layer 4 is in contact with the anode active material layer32.

(Cathode Collecting Electrode 21)

The cathode collecting electrode 21 is formed of an electron conductorwhich causes no chemical change with an ion conductor contained in thecathode active material layer 22 within a range of the predeterminedoperating voltage of the entire solid lithium secondary battery 1. Anexample of the range of the operating voltage of the cathode collectingelectrode 21 is +3 volts to +4.2 volts with respect to a standard redoxpotential of lithium.

The cathode collecting electrode 21 has a shape of a layer when viewedin the cross-sectional view. An example of a material of the cathodecollecting electrode 21 is stainless steel, aluminum, an aluminum alloy,platinum, gold, or titanium. From the viewpoints of the conductivity,the resistance property against the ion conductor, and the redoxpotential of the material, an example of the desirable material of thecathode collecting electrode 21 is aluminum, an aluminum alloy,platinum, or gold. Another example of the material of the cathodecollecting electrode 21 is (i) conductive SrTiO₃ (hereinafter, referredto as “STO”) which has been doped with La or Nb, or (ii) a metalcontaining Pt which has been epitaxially grown on a substrate such as aMgO substrate or a Si substrate. In a case where the cathode activematerial layer 22 which will be described later is epitaxially grown onthe cathode collecting electrode 21, it is desirable that the cathodecollecting electrode 21 is a substrate formed of STO or a substrate onwhich a Pt layer has been epitaxially grown.

(Cathode Active Material Layer 22)

The cathode active material layer 22 contains a cathode active materialcapable of storing and releasing lithium ions. An example of the cathodeactive material is LiCo_(1-a-b)Ni_(a)Al_(b)O₂ (0≦a≦1, 0≦b≦1, and a+b≦1),LiMn₂O₄, or LiFePO₄. The cathode active material layer 22 may containtwo or more kinds of the cathode active materials.

It is desirable that the cathode active material layer 22 iscrystalline. In other words, it is desirable that the cathode activematerial layer 22 is formed of a crystal. The cathode active materiallayer 22 may be an oriented film grown on the cathode collectingelectrode 21 or the solid electrolyte layer 4. More specifically, thecathode active material layer 22 may be a film which has beenepitaxially grown on the cathode collecting electrode 21 or the solidelectrolyte layer 4. Alternatively, the cathode active material layer 22may be formed as below. First, the cathode active material layer 22 isepitaxially grown on a substrate. Then, the cathode active materiallayer 22 is removed from the substrate. Finally, the cathode activematerial layer 22 is disposed on the cathode collecting electrode 21 orthe solid electrolyte layer 4.

The cathode active material layer 22 may contain a conductive assistantand/or a binder.

(Solid Electrolyte Layer 4)

The solid electrolyte layer 4 is formed of a Li_((1-x))TaO₃ crystalhaving a trigonal ilmenite crystal structure. The following mathematicalformula (I) is satisfied.0.12≦x≦0.46   (I)

Lithium ions travel through the solid electrolyte layer 4.

The Li_((1-x))TaO₃ crystal having a value of x less than 0.12 has a lowlithium ion conductivity and a poor charge-discharge property. See thecomparative example 1 (x=0) which will be described later.

It is difficult to form the Li_((1-x))TaO₃ crystal having a value of xmore than 0.46.

Each of a LiTaO₃ crystal and a LiNbO₃ crystal has a trigonal ilmenitecrystal structure. Lithium ions do not travel through conventionalcrystalline LiNbO₃ at room temperature. Specifically, the ionconductivity of lithium ions in the conventional crystalline LiNbO₃ isless than 10⁻⁶ S/cm. As just described, the conventional crystallineLiNbO₃ is unsuitable for use as an electrolyte of a lithium secondarybattery. For a similar reason, the conventional crystalline LiTaO₃ isunsuitable for use as an electrolyte of a lithium secondary battery,either.

As disclosed in A. M. Glass et al., “Ionic conductivity of quenchedalkali niobate and tantalite glasses”, Journal of Applied Physics,49(9), 1978, pp. 4808-4811, amorphous LiNbO₃ has a higher lithium ionconductivity than crystalline LiNbO₃. For this reason, amorphous LiTaO₃is expected to have a higher lithium ion conductivity than crystallineLiTaO₃. However, the present inventors found that amorphous LiTaO₃ has alow lithium ion conductivity. See the comparative example 4 which willbe described later.

Besides, a lithium secondary battery comprising a solid electrolytelayer formed of amorphous LiTaO₃ has a poor charge-discharge propertyand a poor output property. The present inventors believe that this isbecause the battery has high internal resistance. In particular, thepresent inventors believe that this is because an interface resistanceformed between an amorphous LiTaO₃ electrolyte layer and a crystallinecathode active material layer is high. On the other hand, in the presentinvention, the solid electrolyte layer 4 is formed of a Li_((1-x))TaO₃crystal having a trigonal ilmenite crystal structure. The solidelectrolyte layer 4 having the Li_((1-x))TaO₃ crystal having thetrigonal ilmenite crystal structure has a higher lithium ionconductivity than a conventional LiTaO₃ solid electrolyte containingamorphous LiTaO₃. Specifically, for example, the solid electrolyte layer4 has a lithium ion conductivity of not less than 10⁻⁶ S/cm. Since theentire solid lithium secondary battery according to the presentembodiment comprises the solid electrolyte layer 4 having theLi_((1-x))TaO₃ crystal having a trigonal ilmenite crystal structure, theentire solid lithium secondary battery according to the presentembodiment has low internal resistance and a good charge-dischargeproperty.

The crystal structure of LiTaO₃ is a trigonal ilmenite crystalstructure. However, the trigonal ilmenite crystal structure of LiTaO₃ isnot strictly identical to the crystal structure of an ilmenite mineral(for example, titanic iron represented by the chemical formula FeTiO₃).Specifically, in the crystal structure of ilmenite mineral ABO₃, an Alayer, a B layer, and an O layer are stacked along the c-axis directionthereof in the order of the A layer—the A layer—the O layer—the Blayer—the B layer—the O layer. On the other hand, in the trigonalilmenite crystal structure of LiTaO₃, a Li layer, a Ta layer, and an Olayer are stacked along the c-axis direction thereof in the order of theLi layer—the Ta layer—the O layer—the Li layer—the Ta layer—the O layer.The composition of the Li_((1-x))TaO₃ crystal having the trigonalilmenite crystal structure can be confirmed, for example, by acomposition analysis method such as an inductively-coupled plasmaspectrometry. The crystal structure of the Li_((1-x))TaO₃ crystal havingthe trigonal ilmenite crystal structure can be confirmed by a wide-angleX-ray diffraction measurement (hereinafter, referred to as “WAXD”).Specifically, when a diffraction peak is observed near at least onediffraction angle 2θ selected from the group consisting of 23.7 degrees,32.8 degrees, 34.8 degrees, 39.2 degrees, and 62.4 degrees in a WAXDprofile, the crystal is a Li_((1-x))TaO₃ crystal having a trigonalilmenite crystal structure.

The ratio of Li/Ta (atomic ratio) in the Li_((1-x))TaO₃ crystal is notless than 0.54 and not more than 0.88. In other words, the value of(1−x) is not less than 0.54 and not more than 0.88. Therefore, the valueof x is not less than 0.12 and not more than 0.46. The composition ofoxygen included in the Li_((1-x))TaO₃ crystal is slightly deviated fromthe stoichiometric composition of LiTaO₃, depending on the Li/Ta ratio.Specifically, the O/Ta ratio (atomic ratio) in the Li_((1-x))TaO₃crystal is not less than 2.75 and not more than 2.95.

The solid electrolyte layer 4 is formed of the Li_((1-x))TaO₃ crystalhaving a trigonal ilmenite crystal structure. The solid electrolytelayer 4 may be a single-crystalline layer formed of a Li_((1-x))TaO₃single crystal or a polycrystalline layer formed of a Li_((1-x))TaO₃polycrystal. The solid electrolyte layer 4 may slightly contain acrystal structure other than that of the Li_((1-x))TaO₃ crystal. Thecrystal structure other than that of the Li_((1-x))TaO₃ crystal may begenerated when a thick solid electrolyte layer 4 is formed. Theacceptable amount of the crystal structure other than that of theLi_((1-x))TaO₃ crystal included in the solid electrolyte layer 4 is anamount such that the peak intensity derived from the crystal structureother than that of the Li_((1-x))TaO₃ crystal is not more thanone-twentieth the peak intensity derived from the crystal structure ofthe Li_((1-x))TaO₃ crystal in the WAXD profile of the solid electrolytelayer 4.

The Li_((1-x))TaO₃ crystal which forms the solid electrolyte layer 4 mayhave a three-dimensionally random crystal orientation. However,desirably, the Li_((1-x))TaO₃ crystal is oriented along at least onedirection, For example, the Li_((1-x))TaO₃ crystal is oriented along thenormal direction of the solid electrolyte layer 4. In this case, theorientation direction of the Li_((1-x))TaO₃ crystal is the same as thedirection (hereinafter, referred to as “charge-discharge direction”) inwhich the lithium ions travel through the solid electrolyte layer 4during the charge or discharge of the entire solid lithium secondarybattery 1 comprising the solid electrolyte layer 4. This improves theconductivity of lithium ions included in the solid electrolyte layer 4along the charge-discharge direction. As a result, improved are theoutput property and the charge-discharge property of the entire solidlithium secondary battery 1.

The Li_((1-x))TaO₃ crystal may be uniaxially or biaxially oriented. Forexample, a biaxially-oriented Li_((1-x))TaO₃ crystal is oriented alongthe normal direction of the solid electrolyte layer 4 (namely, thecharge-discharge direction) and the in-plane direction of the solidelectrolyte layer 4 (namely, the direction perpendicular to thecharge-discharge direction).

It is desirable that the c-plane of the Li_((1-x))TaO₃ crystal, namely,the (001) plane, is oriented parallel to the charge-discharge direction.In this case, the solid electrolyte layer 4 has a higher lithium ionconductivity along the charge-discharge direction. Specifically, thesolid electrolyte layer 4 can be formed of a Li_((1-x))TaO₃ crystaloriented along a [110] direction or a Li_((1-x))TaO₃ crystal orientedalong a [100] direction. Alternatively, the solid electrolyte layer 4may be formed of the Li_((1-x))TaO₃ crystal oriented along a [−421],[241], or [2−21] direction.

The Li_((1-x))TaO₃ crystal oriented uniaxially or biaxially can beconfirmed by a wide-angle X-ray diffraction method or an electron beamdiffraction method. For example, in the Li_((1-x))TaO₃ crystal orienteduniaxially or biaxially, only a diffraction peak of a specific planedirection or only integral multiple diffraction peaks thereof is/areobserved in a θ−2θ method of the wide-angle X-ray diffraction method.Furthermore, in a case of the Li_((1-x))TaO₃ crystal oriented biaxially,a diffraction peak is observed in a φ-scan method of the X-raydiffraction method. The peak interval accords with a rotational symmetryof the plane direction observed in the θ−2θ method. On the other hand,in the Li_((1-x))TaO₃ crystal oriented uniaxially, no peak is observedin the φ-scan method. In the electron beam diffraction method, atransmission electron microscope (hereinafter, referred to as “TEM”) maybe used.

The thickness of the solid electrolyte layer 4 is not limited. However,if the solid electrolyte layer 4 is too thin, an electrical short may beformed, since a pinhole is generated in the solid electrolyte layer 4.On the other hand, if the solid electrolyte layer 4 is too thick, theoutput property of the entire solid lithium secondary battery 1 isdeteriorated, since the resistance with respect to the migration oflithium ions is high. The solid electrolyte layer 4 desirably has athickness of approximately not less than 100 nanometers and not morethan 20 micrometers. The solid electrolyte layer 4 having a thicknessfalling within this range lowers the resistance value per unit area ofthe solid electrolyte layer 4 (namely, the resistance value with respectto the migration of lithium ions) to approximately 50 ohm cm² or less.More desirably, the solid electrolyte layer 4 has a thickness ofapproximately not less than 200 nanometers and not more than 2micrometers.

The method for forming the solid electrolyte layer 4 is not limited. Thesolid electrolyte layer 4 may be formed by a known method for forming athin film such as a pulse laser deposition method (hereinafter, referredto as “PLD method”), a vacuum deposition method, a sputtering method, achemical vapor deposition method, or a sol-gel method. The solidelectrolyte layer 4 may be epitaxially grown on the cathode activematerial layer 22 or the anode active material layer 32. The solidelectrolyte layer 4 is formed of the LiTaO₃ crystal having a trigonalilmenite crystal structure in which lithium atoms are deficientsignificantly. For this reason, employed is a method for causing lithiumatoms to be deficient by heating the LiTaO₃ crystal layer fabricated bya sintering method.

(Anode Active Material Layer 32)

The anode active material layer 32 may contain the anode active materialcapable of storing and releasing lithium ions. The anode active materialis capable of storing and releasing lithium ions at a lower potentialthan the cathode active material. An example of the anode activematerial is a lithium alloy, an alloy, an intermetallic compound,carbon, an organic compound, an inorganic compound, a metal complex, oran organic polymer compound. The anode active material layer 32 maycontain two or more kinds of these substances. The anode active materiallayer 32 may contain a conductive assistant and/or a binder.

It is desirable that the anode active material layer 32 is crystalline.In other words, it is desirable that the anode active material layer 32is formed of a crystal. The anode active material layer 32 may be anoriented film grown on the anode collecting electrode 31 or the solidelectrolyte layer 4. More specifically, the anode active material layer32 may be a film which has been epitaxially grown on the anodecollecting electrode 31 or the solid electrolyte layer 4. Alternatively,the anode active material layer 32 may be formed as below. First, theanode active material layer 32 is epitaxially grown on a substrate.Then, the substrate is removed from the anode active material layer 32.Finally, the anode active material layer 32 is disposed on the anodecollecting electrode 31 or the solid electrolyte layer 4.

(Anode Collecting Electrode 31)

The anode collecting electrode 31 is formed of an electron conductorwhich causes no chemical change with an ion conductor contained in theanode active material layer 32 within a range of the predeterminedoperating voltage of the entire solid lithium secondary battery 1. Anexample of the range of the operating voltage of the anode collectingelectrode 31 is 0 volts to +1.6 volts with respect to a standardoxidation-reduction potential of lithium.

The anode collecting electrode 31 has a shape of a layer when viewed inthe cross-sectional view. An example of a material of the anodecollecting electrode 31 is stainless steel, nickel, copper, titanium,aluminum, an aluminum alloy, platinum, or gold. From the viewpoints ofthe conductivity, the resistance property against the ion conductor, andthe oxidation-reduction potential of the material, an example of thedesirable material of the anode collecting electrode 31 is aluminum, analuminum alloy, platinum, or gold.

The anode active material layer 32 is disposed on the anode collectingelectrode 31. In a case where the anode active material layer 32 isepitaxially grown on the anode collecting electrode 31, it is desirablethat the anode collecting electrode 31 is a conductive STO substratedoped with La or Nb.

(Fabrication Method)

Hereinafter, a method for fabricating the entire solid lithium secondarybattery according to the present embodiment will be described withreference to FIG. 2A to FIG. 2D.

First, as shown in FIG. 2A, the cathode active material layer 22 isformed on the cathode collecting electrode 21 by a sputtering method, avacuum deposition method, a CVD method, a PLO method, or a sol-gelmethod. In this way, the cathode 2 is formed.

Then, as shown in FIG. 2B, the solid electrolyte layer 4 is formed onthe cathode active material layer 22. As described above, it isdesirable that the solid electrolyte layer 4 has a crystal structureoriented along a predetermined direction. The requirement for formingsuch a solid electrolyte layer 4, especially, the requirement forcontrolling the crystal orientation, is configured appropriatelydepending on a specific method for forming the solid electrolyte layer4.

Next, as shown in FIG. 2C, the anode active material layer 32 is formedon the solid electrolyte layer 4 by a sputtering method, a vacuumdeposition method, a CVD method, a PLD method, or a sol-gel method.

Finally, as shown in FIG. 2D, the anode collection electrode 31 isformed on the anode active material layer 32 by a sputtering method, avacuum deposition method, a CVD method, a PLD method, or a sol-gelmethod to give the anode 3. Alternatively, a metal foil may be disposedas the anode collection electrode 31 on the anode active material layer32. In this way, the entire solid lithium secondary battery 1 accordingto the present embodiment is provided.

The method for fabricating the entire solid lithium secondary battery 1is not limited to the embodiment shown in FIG. 2A-FIG. 2D. For example,the entire solid lithium secondary battery 1 may be obtained by formingthe anode active material layer 32, the solid electrolyte layer 4, thecathode active material layer 22, and the cathode collecting electrode21 in this order.

EXAMPLES

Hereinafter, the present invention will be described in more detail withreference to the examples.

Inventive Example 1

[Fabrication of the Battery]

An entire solid lithium secondary battery 1 was fabricated in accordancewith the method shown in FIG. 2A-FIG. 2D. Specifically, the entire solidlithium secondary battery 1 was fabricated as below.

A STO single-crystal substrate doped with La was prepared as the cathodecollecting electrode 21. The STO substrate had a principal surfacehaving a size of 10 millimeters×10 millimeters. The STO substrate had athickness of 500 micrometers. The STO substrate had a principal surfaceof a (110) plane.

Then, as shown in FIG. 2A, the cathode active material layer 22 formedof lithium cobalt oxide represented by the chemical formula LiCoO₂ wasformed on the principal surface of the cathode collecting electrode 21by a PLD method. The cathode active material layer 22 had a thickness of250 nanometers. The condition of the PLD method for forming the cathodeactive material layer 22 is described below.

Target: Sintered oxide containing Li and Co (Li:Co=1.3:1 (atomic ratio))

Energy of laser pulse: 50 mJ

Pulse frequency: 2.5 Hz

Partial pressure of oxygen: 3 Pa

Temperature of substrate: 500 degrees Celsius

Interval between the target and the substrate: 55 millimeters

Next, as shown in FIG. 2B, the solid electrolyte layer 4 was formed onthe surface of the cathode active material layer 22 by a PLD method. Thesolid electrolyte layer 4 had a thickness of 200 nanometers. Thecondition of the PLD method for forming the solid electrolyte layer 4 isdescribed below.

Target: Sintered oxide containing Li and Ta (Li:Ta=1.3:1 (atomic ratio))

Energy of laser pulse: 100 mJ

Pulse frequency: 5 Hz

Partial pressure of oxygen: 10 Pa

Temperature of substrate: 500 degrees Celsius

Interval between the target and the substrate: 40 millimeters

[Characterization of Crystal Structure and Crystal Orientation]

After the formation of the solid electrolyte layer 4, the crystalstructures of the cathode active material layer 22 and the solidelectrolyte layer 4 were analyzed by a wide-angle X-ray diffractionmethod using an X-ray diffraction device (available from Philips Co.,Ltd., Trade name: X'Pert MRD). FIG. 3 shows a diffraction profile. Asshown in FIG. 3, a diffraction peak derived from a (110) plane of theLiCoO₂ crystal was observed. This diffraction peak corresponded to thecathode active material layer 22. Also observed was a diffraction peakderived from a (300) plane of the LiTaO₃ crystal having a trigonalilmenite crystal structure. This diffraction peak corresponded to thesolid electrolyte layer 4. Diffraction peaks other than these twodiffraction peaks had a significantly small intensity. For example, apeak derived from a (006) plane had an intensity less than one-twentieththat of the peak derived from the (300) plane. In order to observe thecrystal orientation in the in-plane direction of the solid electrolytelayer 4, the φ-scan of a (012) plane was conducted. As a result, atwo-fold symmetry was observed. No diffraction peak of monoclinicLiTa₃O₈ was observed.

Next, as shown in FIG. 2C, the anode active material layer 32 formed oflithium was formed on the surface of the solid electrolyte layer 4 by asputtering method. The anode active material layer 32 had a thickness of200 nanometers.

Finally, as shown in FIG. 2D, the anode collecting electrode 31 formedof platinum was formed on the surface of the anode active material layer32 by a sputtering method. The anode collecting electrode 31 had athickness of 100 nanometers. In this way, as shown in FIG. 1, the entiresolid lithium secondary battery according to the inventive example 1 wasprovided.

[Measurement of the Capacitance Ratio]

The capacitance ratio of the entire solid lithium secondary battery 1according to the inventive example 1 was measured as below. First, apotentiostat was connected to the cathode collecting electrode 21 andthe anode collecting electrode 31 under an argon atmosphere. Next, theentire solid lithium secondary battery 1 according to the inventiveexample 1 was charged to 4.2 volts using a constant electric current of60 microamperes output from the potentiostat. Then, the entire solidlithium secondary battery 1 according to the inventive example 1 wasdischarged to 3.0 volts at 60 microamperes. The discharged capacitanceat 60 microamperes was defined as “1 C”.

The potentiostat was connected again, and the entire solid lithiumsecondary battery 1 according to the inventive example 1 was charged to4.2 volts using the constant electric current of 60 microamperes. Next,the entire solid lithium secondary battery 1 according to the inventiveexample 1 was discharged to 3.0 volts at 120 microamperes. Thedischarged capacitance at 120 microamperes was defined as “2 C”. Thecapacitance ratio is represented by 2 C/1 C. The entire solid lithiumsecondary battery 1 according to the inventive example 1 had acapacitance ratio of (2 C/1 C) of 0.61. The capacitance ratio (2 C/1 C)means the amount of change of the capacitance of the battery when thecharge-discharge rate is changed. With an increase in the capacitanceratio (2 C/1 C), the battery has a lower internal resistance, a bettercharge-discharge property, and a better output property.

[Measurement of the Conductivity of Lithium Ions]

A sample for evaluation was prepared as below. First, a non-dopedinsulative STO substrate was prepared. The insulative STO substrate hasa principal surface of a (110) plane and a size of 10 millimeters×10millimeters×500 micrometers. Next, as shown in FIG. 2B, the solidelectrolyte layer 4 was formed on the insulative STO substrate in a waysimilar to the case of the fabrication of the entire solid lithiumsecondary battery 1. In this way, the sample for evaluation wasfabricated. The sample for evaluation did not have the cathode activematerial layer 22 and the anode 3.

Next, using an inductively-coupled plasma optical emission spectrometer(available from Hitachi High-Tech Science Corporation, Trade name:SPS1700VR, hereinafter, referred to as “ICP analysis device”), thecomposition of the solid electrolyte layer 4 included in the sample forevaluation was identified. As a result, the Li:Ta ratio was equal to0.88:1 (atomic ratio) in the solid electrolyte layer 4. In other words,the value of x was equal to 0.12.

Two silver electrodes were formed on the surface of the solidelectrolyte layer 4 included in the sample for evaluation. One electrodeserved as the cathode. The other electrode served as the anode. Then,the cathode and the anode of a potentiostat (available from PrincetonApplied Research, Trade name: VersaSTAT4) were connected to these silverelectrodes. An AC impedance with respect to the solid electrolyte layer4 was measured. The measurement range was 1 Hz to 1 MHz. In this way,the resistance value with respect to travel of the lithium ions alongthe in-plane direction of the solid electrolyte layer 4 was measured.The measured resistance value was converted into the lithium ionconductivity. The lithium ion conductivity (in-plane direction) of thesolid electrolyte layer 4 according to the inventive example 1 was2.1×10⁻⁶ S/cm.

Inventive Example 2

An entire solid lithium secondary battery 1 was fabricated similarly tothe case of the inventive example 1, except that the condition of thePLD method for forming the solid electrolyte layer 4 was as below.

Target: Sintered oxide containing Li and Ta (Li:Ta=1.3:1 (atomic ratio))

Energy of laser pulse: 100 mJ

Pulse frequency: 2 Hz

Partial pressure of oxygen: 10 Pa

Temperature of substrate: 700 degrees Celsius

Interval between the target and the substrate: 40 millimeters

The entire solid lithium secondary battery 1 according to the inventiveexample 2 had a capacitance ratio (2 C/1 C) of 0.60. After the formationof the solid electrolyte layer 4, the crystalline structures of thecathode active material layer 22 and the solid electrolyte layer 4 wereanalyzed by a wide-angle X-ray diffraction method similarly to the caseof the inventive example 1. As a result, a diffraction peak derived froma (110) plane of the LiCoO₂ crystal was observed. This diffraction peakcorresponded to the cathode active material layer 22. Also observed wasa diffraction peak derived from a (110) plane of the LiTaO₃ crystalhaving a trigonal ilmenite crystal structure. This diffraction peakcorresponded to the solid electrolyte layer 4. Diffraction peaks otherthan these two diffraction peaks had a significantly small intensity.For example, a peak derived from a (006) plane had a less thanone-twentieth times smaller intensity than a peak derived from a (110)plane. In order to confirm the in-plane crystal orientation of the solidelectrolyte layer 4 in the inventive example 2, the φ-scan of the (104)plane was conducted. As a result, a two-fold symmetry was confirmed. Nodiffraction peak of monoclinic LiTa₃O₈ was observed.

Similarly to the case of the inventive example 1, the composition andthe in-plane lithium ion conductivity of the solid electrolyte layer 4in the inventive example 2 were measured. As a result, the Li:Ta ratiowas equal to 0.75:1 (atomic ratio) in the solid electrolyte layer 4 inthe inventive example 2. In other words, the value of x was equal to0.25. The lithium ion conductivity was 1.2×10⁻⁵ S/cm.

Inventive Example 3

An entire solid lithium secondary battery 1 was fabricated similarly tothe case of the inventive example 1, except that the condition of thePLD method for forming the solid electrolyte layer 4 was as below.

Target: Sintered oxide containing Li acid Ta (Li:Ta=1.3:1 (atomicratio))

Energy of laser pulse: 100 mJ

Pulse frequency: 2 Hz

Partial pressure of oxygen: 10 Pa

Temperature of substrate: 600 degrees Celsius

Interval between the target and the substrate: 50 millimeters

The entire solid lithium secondary battery 1 according to the inventiveexample 3 had a capacitance ratio (2 C/1 C) of 0.63. After the formationof the solid electrolyte layer 4, the crystalline structures of thecathode active material layer 22 and the solid electrolyte layer 4 wereanalyzed by a wide-angle X-ray diffraction method similarly to the caseof the inventive example 1. As a result, a diffraction peak derived froma (110) plane of the LiCoO₂ crystal was observed. This diffraction peakcorresponded to the cathode active material layer 22. Also observed wasa diffraction peak derived from a (300) plane of the LiTaO₃ crystalhaving a trigonal ilmenite crystal structure. This diffraction peakcorresponded to the solid electrolyte layer 4. Diffraction peaks otherthan these two diffraction peaks had a significantly small intensity.For example, a peak derived from a (006) plane had a less thanone-twentieth times smaller intensity than a peak derived from a (300)plane. In order to confirm the in-plane crystal orientation of the solidelectrolyte layer 4 in the inventive example 3, the φ-scan of the (012)plane was conducted. As a result, a two-fold symmetry was confirmed. Nodiffraction peak of monoclinic LiTa₃O₈ was observed.

Similarly to the case of the inventive example 1, the composition andthe in-plane lithium ion conductivity of the solid electrolyte layer 4in the inventive example 3 were measured. As a result, the Li:Ta ratiowas equal to 0.54:1 (atomic ratio) in the solid electrolyte layer 4 inthe inventive example 3. In other words, the value of x was equal to0.46. The lithium ion conductivity was 4.5×10⁻⁵ S/cm.

Inventive Example 4

An entire solid lithium secondary battery 1 was fabricated similarly tothe case of the inventive example 1, except that the plane direction ofthe STO substrate was a (111) plane and that the condition of the PLDmethod for forming the solid electrolyte layer 4 was as below.

Target: Sintered oxide containing Li and Ta (Li:Ta=1.0:1 (atomic ratio))

Energy of laser pulse: 100 mJ

Pulse frequency: 2 Hz

Partial pressure of oxygen: 10 Pa

Temperature of substrate: 600 degrees Celsius

Interval between the target and the substrate: 40 millimeters

The entire solid lithium secondary battery 1 according to the inventiveexample 4 had a capacitance ratio (2 C/1 C) of 0.63. After the formationof the solid electrolyte layer 4, the crystalline structures of thecathode active material layer 22 and the solid electrolyte layer 4 wereanalyzed by a wide-angle X-ray diffraction method similarly to the caseof the inventive example 1. As a result, a diffraction peak derived froma (003) plane of the LiCoO₂ crystal was observed. This diffraction peakcorresponded to the cathode active material layer 22. Also observed wasa diffraction peak derived from a (006) plane of the LiTaO₃ crystalhaving a trigonal ilmenite crystal structure. This diffraction peakcorresponded to the solid electrolyte layer 4. Diffraction peaks otherthan these two diffraction peaks had a significantly small intensity.For example, a peak derived from a (300) plane had a less thanone-twentieth times smaller intensity than a peak derived from a (006)plane. In order to confirm the in-plane crystal orientation of the solidelectrolyte layer 4 in the inventive example 4, the φ-scan of the (104)plane was conducted. As a result, a six-fold symmetry was confirmed. Nodiffraction peak of monoclinic LiTa₃O₈ was observed.

Similarly to the case of the inventive example 1, the composition andthe in-plane lithium ion conductivity of the solid electrolyte layer 4in the inventive example 4 were measured. As a result, the Li:Ta ratiowas equal to 0.62:1 (atomic ratio) in the solid electrolyte layer 4 inthe inventive example 4. In other words, the value of x was equal to0.38. The lithium ion conductivity was 57×10⁻⁵ S/cm.

Inventive Example 5

An entire solid lithium secondary battery 1 was fabricated similarly tothe case of the inventive example 4, except that the solid electrolytelayer 4 had a thickness of 2200 nanometers.

The entire solid lithium secondary battery 1 according to the inventiveexample 5 had a capacitance ratio (2 C/1 C) of 0.60. After the formationof the solid electrolyte layer 4, the crystalline structures of thecathode active material layer 22 and the solid electrolyte layer 4 wereanalyzed by a wide-angle X-ray diffraction method similarly to the caseof the inventive example 1. As a result, a diffraction peak derived froma (003) plane of the LiCoO₂ crystal was observed. This diffraction peakcorresponded to the cathode active material layer 22. Also observed wasa diffraction peak derived from a (006) plane of the LiTaO₃ crystalhaving a trigonal ilmenite crystal structure. This diffraction peakcorresponded to the solid electrolyte layer 4. Diffraction peaks otherthan these two diffraction peaks had a significantly small intensity.For example, a peak derived from a (300) plane had a less thanone-twentieth times smaller intensity than a peak derived from a (006)plane. In order to confirm the in-plane crystal orientation of the solidelectrolyte layer 4 in the inventive example 5, the φ-scan of the (104)plane was conducted. As a result, a six-fold symmetry was confirmed. Nodiffraction peak of monoclinic LiTa₃O₈ was observed.

Similarly to the case of the inventive example 1, the composition andthe in-plane lithium ion conductivity of the solid electrolyte layer 4in the inventive example 5 were measured. As a result, the Li:Ta ratiowas equal to 0.62:1 (atomic ratio) in the solid electrolyte layer 4 inthe inventive example 5. In other words, the value of x was equal to0.38. The lithium ion conductivity was 3.2×10⁻⁵ S/cm.

Comparative Example 1

An entire solid lithium secondary battery 1 was fabricated similarly tothe case of the inventive example 1, except that the condition of thePLD method for forming the solid electrolyte layer 4 was as below.

Target: Sintered oxide containing Li and Ta (Li:Ta=1.5:1 (atomic ratio))

Energy of laser pulse: 100 mJ

Pulse frequency: 5 Hz

Partial pressure of oxygen: 10 Pa

Temperature of substrate: 450 degrees Celsius

Interval between the target and the substrate: 40 millimeters

The entire solid lithium secondary battery 1 according to thecomparative example 1 had a capacitance ratio (2 C/1 C) of 0.42. Afterthe formation of the solid electrolyte layer 4, the crystallinestructures of the cathode active material layer 22 and the solidelectrolyte layer 4 were analyzed by a wide-angle X-ray diffractionmethod similarly to the case of the inventive example 1. As a result, adiffraction peak derived from a (110) plane of the LiCoO₂ crystal wasobserved. This diffraction peak corresponded to the cathode activematerial layer 22. Also observed was a diffraction peak derived from a(300) plane of the LiTaO₃ crystal having a trigonal ilmenite crystalstructure. This diffraction peak corresponded to the solid electrolytelayer 4. Diffraction peaks other than these two diffraction peaks had asignificantly small intensity. For example, a peak derived from a (006)plane had a less than one-twentieth times smaller intensity than a peakderived from a (300) plane. In order to confirm the in-plane crystalorientation of the solid electrolyte layer 4 in the comparative example1, the φ-scan of the (104) plane was conducted. As a result, a two-foldsymmetry was confirmed. No diffraction peak of monoclinic LiTa₃O₈ wasobserved.

Similarly to the case of the inventive example 1, the composition andthe in-plane lithium ion conductivity of the solid electrolyte layer 4in the comparative example 1 were measured. As a result, the Li:Ta ratiowas 1.00:1 (atomic ratio) in the solid electrolyte layer 4 in thecomparative example 1. In other words, the value of x was equal to 0.00.The lithium ion conductivity was 2.0×10⁻⁷ S/cm.

Comparative Example 2

An entire solid lithium secondary battery 1 was fabricated similarly tothe case of the inventive example 1, except that the condition of thePLD method for forming the solid electrolyte layer 4 was as below.

Target: Sintered oxide containing Li and Ta (Li:Ta=1:1 (atomic ratio))

Energy of laser pulse: 100 mJ

Pulse frequency: 2 Hz

Partial pressure of oxygen: 10 Pa

Temperature of substrate: 250 degrees Celsius

Interval between the target and the substrate: 40 millimeters

The entire solid lithium secondary battery 1 according to thecomparative example 2 had a capacitance ratio (2 C/1 C) of 0.32. Afterthe formation of the solid electrolyte layer 4, the crystal structuresof the cathode active material layer 22 and the solid electrolyte layer4 were analyzed similarly to the case of the inventive example 1. FIG. 4shows a diffraction profile. As is clear from FIG. 4, a diffraction peakderived from a (110) plane of the LiCoO₂ crystal was observed. Thiscorresponds to the cathode active material layer 22. Furthermore,observed was a diffraction peak derived from a (012) plane of the LiTaO₃crystal having a trigonal ilmenite crystal structure. This correspondsto the solid electrolyte layer 4. Besides, various diffraction peakswere observed, including diffraction peaks which correspond to a (006)plane and a (300) plane of the LiTaO₃ crystal. In other words, the solidelectrolyte layer 4 was polycrystalline. A diffraction peak ofmonoclinic LiTa₃O₃ was also observed.

Similarly to the case of the inventive example 1, the composition andthe in-plane lithium ion conductivity of the solid electrolyte layer 4in the comparative example 2 were measured. As a result, the Li:Ta ratiowas equal to 0.85:1 (atomic ratio) in the solid electrolyte layer 4 inthe comparative example 2. In other words, the value of x was equal to0.15. The lithium ion conductivity was less than 1.0×10⁻⁸ S/cm.

Comparative Example 3

An entire solid lithium secondary battery 1 was fabricated similarly tothe case of the inventive example 1, except that the condition of thePLD method for forming the solid electrolyte layer 4 was as below.

Target: Sintered oxide containing Li and Ta (Li:Ta=1.3:1 (atomic ratio))

Energy of laser pulse: 100 mJ

Pulse frequency: 2 Hz

Partial pressure of oxygen: 10 Pa

Temperature of substrate: 250 degrees Celsius

Interval between the target and the substrate: 40 millimeters

The entire solid lithium secondary battery 1 according to thecomparative example 3 had a capacitance ratio (2 C/1 C) of 0.37. Afterthe formation of the solid electrolyte layer 4, the crystal structuresof the cathode active material layer 22 and the solid electrolyte layer4 were analyzed similarly to the case of the inventive example 1.Similarly to the case of the comparative example 2, a diffraction peakderived from a (110) plane of the LiCoO₂ crystal was observed. Thiscorresponds to the cathode active material layer 22. Furthermore,observed was a diffraction peak derived from a (012) plane of the LiTaO₃crystal having a trigonal ilmenite crystal structure. This correspondsto the solid electrolyte layer 4. Besides, various diffraction peakswere observed, including diffraction peaks which correspond to a (006)plane and a (300) plane of the LiTaO₃ crystal. In other words, the solidelectrolyte layer 4 was polycrystalline. A diffraction peak ofmonoclinic LiTa₃O₃ was also observed.

Similarly to the case of the inventive example 1, the composition andthe in-plane lithium ion conductivity of the solid electrolyte layer 4in the comparative example 3 were measured. As a result, the Li:Ta ratiowas equal to 0.72:1 (atomic ratio) in the solid electrolyte layer 4 inthe comparative example 3. In other words, the value of x was equal to0.28. The lithium ion conductivity was 4.1×10⁻⁸ S/cm.

Comparative Example 4

An entire solid lithium secondary battery 1 was fabricated similarly tothe case of the inventive example 1, except that the solid electrolytelayer 4 was formed not by a PLD method but by a sputtering method. Thecondition of the sputtering method was as below.

Target: Sintered oxide containing Li and Ta(Li:Ta=1:1 (atomic ratio))

RF power: 80 W

Sputtering gas pressure: 1 Pa (Ar:O₂=80:20 (volume ratio))

Substrate temperature: room temperature

The entire solid lithium secondary battery 1 according to thecomparative example 4 had a capacitance ratio (2 C/1 C) of 0.38. Afterthe formation of the solid electrolyte layer 4, the crystal structuresof the cathode active material layer 22 and the solid electrolyte layer4 were analyzed similarly to the case of the inventive example 1. As aresult, a diffraction peak derived from a (110) plane of the LiCoO₂crystal was observed. This corresponds to the cathode active materiallayer 22. However, no diffraction peak derived from the solidelectrolyte layer 4 was observed. In other words, the solid electrolytelayer 4 was amorphous in the comparative example 4.

Similarly to the case of the inventive example 1, the composition andthe in-plane lithium ion conductivity of the solid electrolyte layer 4in the comparative example 4 were measured. As a result, the Li:Ta ratiowas 0.85:1 (atomic ratio) in the solid electrolyte layer 4 in thecomparative example 4. In other words, the value of x was equal to 0.15.The lithium ion conductivity was 6.0×10⁻⁷ S/cm.

The following Table 1 and Table 2 show the results of the inventiveexamples 1-5 and the comparative examples 1-4.

TABLE 1 Orientation of Li/Ta ratio Li_((1−x))TaO₃ Crystal structure(Value of x) crystal Inventive trigonal ilmenite structure 0.88 (100)example 1 (x = 0.12) Inventive trigonal ilmenite structure 0.75 (110)example 2 (x = 0.25) Inventive trigonal ilmenite structure 0.54 (100)example 3 (x = 0.46) Inventive trigonal ilmenite structure 0.62 (001)example 4 (x = 0.38) Inventive trigonal ilmenite structure 0.62 (001)example 5 (x = 0.38) Comparative trigonal ilmenite structure 1.00 (100)example 1 (x = 0.00) Comparative mixed structure of trigonal 0.85polycrystalline example 2 ilmenite structure and (x = 0.15) monocliniccrystal Comparative mixed structure of trigonal 0.72 polycrystallineexample 3 ilmenite structure and (x = 0.28) monoclinic crystalComparative amorphous 0.85 amorphous example 4 (x = 0.15)

TABLE 2 Lithium ion conductivity Capacitance ratio (S/cm) (2 C/1 C)Inventive 2.1 × 10⁻⁶ 0.61 example 1 Inventive 1.2 × 10⁻⁵ 0.60 example 2Inventive 4.5 × 10⁻⁵ 0.63 example 3 Inventive 5.7 × 10⁻⁵ 0.63 example 4Inventive 3.2 × 10⁻⁵ 0.60 example 5 Comparative 2.0 × 10⁻⁷ 0.42 example1 Comparative Less than 1.0 × 10⁻⁸ 0.32 example 2 Comparative 4.1 × 10⁻⁸0.37 example 3 Comparative 6.0 × 10⁻⁷ 0.38 example 4

As shown in Table 1 and Table 2, each of the solid electrolyte layers 4according to the inventive examples 1-5 has a higher lithium ionconductivity than the solid electrolyte layer 4 according to thecomparative example 1 (Li/Ta ratio is 1). In other words, the entiresolid lithium secondary battery 1 comprising the solid electrolyte layer4 having a Li/Ta value of not less than 0.54 and not more than 0.88(namely, x is not less than 0.12 and not more than 0.46) has a higherlithium on conductivity than the entire solid lithium secondary battery1 comprising the solid electrolyte layer 4 having a Li/Ta value of 1(namely, x is equal to 0).

Each of the entire solid lithium secondary batteries 1 according to theinventive examples 1-5 has a large capacitance ratio 2 C/1 C.Accordingly, each of the entire solid lithium secondary batteries 1according to the inventive examples 1-5 has the low internal resistance,the good charge-discharge property, and the good output property. Thesolid electrolyte layer 4 in the inventive example 5 is more than tentimes thicker than those in the inventive examples 1-4. However, theentire solid lithium secondary battery 1 according to the inventiveexample 5 has similar properties to those of the inventive examples 1-4.

The solid electrolyte layer 4 contained a LiTa₃O₈ phase in thecomparative examples 2-3, although the Li/Ta ratio in the solidelectrolyte layer 4 was not more than 0.88 (namely, x was not less than0.12) and the solid electrolyte layer 4 was crystalline. For thisreason, the lithium ion conductivity of the solid electrolyte layer 4was very low in the comparative examples 2-3.

In the comparative example 4, the solid electrolyte layer 4 wasamorphous. The solid electrolyte layer 4 in the comparative example 4had a higher lithium ion conductivity than those of the comparativeexamples 1-3 in which the solid electrolyte layers 4 were crystalline.However, the lithium ion conductivity in the comparative example 4 waslower than those of the inventive examples 1-5. Although the solidelectrolyte layer 4 in the comparative example 4 had a higher lithiumion conductivity than those of the comparative examples 1-3, the solidelectrolyte layer 4 in the comparative example 4 had a capacitance ratio2 C/1 C similar to those of the comparative examples 1-3. This meansthat the entire solid lithium secondary battery 1 according to thecomparative example 4 has higher internal resistance than the entiresolid lithium secondary batteries according to the comparative examples1-3.

INDUSTRIAL APPLICABILITY

The entire solid lithium secondary battery according to the presentinvention can be used, for example, for a power source of a mobiledevice, an electric power tool, or transportation equipment. An exampleof the transportation equipment is an electric vehicle.

REFERENCE SIGNS LIST

-   1 Entire solid lithium secondary battery-   2 Cathode

21 Cathode collecting electrode

22 Cathode active material layer

-   3 Anode

31 Anode collecting electrode

32 Anode active material layer

-   4 Solid electrolyte layer

The invention claimed is:
 1. An entire solid lithium secondary batterycomprising: a cathode; an anode; and a solid electrolyte layer disposedbetween the cathode and the anode; wherein the solid electrolyte layeris formed of a Li_((1-x))TaO₃ crystal (where 0.12≦x≦0.46) having atrigonal ilmenite crystal structure.
 2. The entire solid lithiumsecondary battery according to claim 1, wherein the solid electrolytelayer has a thickness of not less than 100 nanometers and not more than20 micrometers.
 3. The entire solid lithium secondary battery accordingto claim 1, wherein the Li_((1-x))TaO₃ crystal is oriented along anormal direction of the solid electrolyte layer.
 4. The entire solidlithium secondary battery according to claim 1, wherein theLi_((1-x))TaO₃ crystal is biaxially oriented along a normal directionand an in-plane direction of the solid electrolyte layer.
 5. The entiresolid lithium secondary battery according to claim 1, wherein theLi_((1-x))TaO₃ crystal is oriented along a [110] direction or along a[100] direction.